Experiment 86

Synthesis of Esters

Problem

What simple organic compounds can be synthesized in the laboratory?

Introduction

One of the most important commercial (and natural) types of organic syntheses involves the reaction of an organic acid, called a carboxylic acid, with an alcohol to produce an ester. A simple example, shown below, is the reaction between methyl alcohol, CH3OH, and acetic acid, CH3COOH. The reaction is presented with structural formulas to better illustrate the way the reaction takes place, although the actual mechanism is of no interest to us here.

The H+ above the reaction arrow indicates that the reaction requires an acid catalyst. Esters are generally characterized by sweet, often pleasant odors, including many of the familiar fruit aromas. Banana oil, for example, is the result of ester formation, produced by the reaction between acetic acid (shown in the reaction above) and isoamyl alcohol, CH3CH2CH(OH)CH2CH3.

In this experiment, you will make one or more esters, carrying out the reaction on a small scale. Since you are only interested in the nature of the product, and have no concern for the stoichiometry or percentage yield, precise measurements of reactant quantities is not necessary. All that you need to do is to combine the reactants, then allow them to boil gently together for a short time. This process, in which a reaction mixture is maintained at a slow boil, without a loss of material, is known as refluxing. A condenser attached to the top of the reaction vessel allows any evaporating liquid to cool, recondense, and return to the liquid in the tube.

While the goal of the experiment is to make the specific ester methyl salicylate, your teacher may provide the reactants for you to make one or more esters, since the process is quick and simple to carry out. These may be assigned as part of the activity, or they may be offered as extra credit.

Prelaboratory Assignment

Materials

Apparatus

Reagents

Safety

1. Concentrated sulfuric acid is extremely corrosive to skin and clothing. Handle it with great respect, use it only in the fume hood, and use only the dropper that comes with the dispensing bottle. Gloves may be worn to protect your hands when pouring the concentrated acid.

2. The reaction mixtures contain sulfuric acid. Neutralize any spills and clean them up with large amounts of water.

3. Methanol is highly flammable, as are all of the alcohols in the Optional Reagents list. Be sure there are no open flames in the laboratory.

4. Follow safe lab technique when testing the aroma of your product. The odors of some of the esters, although familiar and pleasant, are quite penetrating and harsh in large doses.

5. Safety goggles and a lab apron must be worn at all times in the laboratory.

6. Remember that hot plates and hot glassware look the same as cool objects. Take care to avoid burns.

Procedure

If you are not using a Report Sheet for this experiment be sure to include the following in your Data/Observations:

Initial Appearance and odor of:

Describe what you saw during the refluxing. Could you tell that liquid was being recondensed? If so, what did you see that indicated this was happening?

Appearance and odor of:

1. Make a reflux condenser by wrapping a small piece of Parafilm™ or other laboratory film around the wide end of a Pasteur pipet. Use enough to allow the pipet to fit snugly into the top of a standard 100-mm test tube as shown in Figure 1. Wrapping two or three layers around the pipet usually is enough. Set the condenser aside for a moment. Turn the hotplate on, using the low-power setting.

Figure 1

Reflux assembly

2. Place a small amount of salicylic acid (2-hydroxybenzoic acid, C6H4(OH)COOH) in the tube; use enough to give a depth of about 0.5 cm of the solid acid.

3. Add an approximately equal volume of methyl alcohol; the acid will dissolve in the alcohol, although perhaps not completely at first.

4. Go to the fume hood where you will find a small dropper bottle of concentrated sulfuric acid; this is the catalyst. DANGER: sulfuric acid is very harmful. Using the dropper provided, add 2 or 3 drops of the acid to your reaction mixture, then return the dropper to the bottle. Finally add a boiling chip to the tube, taking care to avoid splashing.

5. Insert the Pasteur pipet/reflux condenser into the top of the reaction tube. Using a wire test tube holder, allow the tube to rest lightly on the surface of the hot plate, in the small hole in the heat transfer block. The mixture should begin to reflux within a minute or two.

You can control the rate at which the refluxing takes place by raising the tube off the surface of the hot plate if boiling becomes too rapid, or by lowering it if the reaction slows below the desired rate.

Maintain the reflux for two or three minutes, then remove the tube to a beaker or other heat-proof holder and allow it to cool for a few minutes.

6. Carefully remove the condenser, then hold the tube about 6 inches away from your face, and wave your hand across the top of the tube to direct the odor of your product toward your nose. The chemical name of the ester you produced is methyl salicylate; you may recognize it as oil of wintergreen.

Note: If you are going to carry out one or more additional esterification reactions, clean up the first one as described in the following section, except do not discard the reflux condenser; it can be used again, so long as the Parafilm™ does not deteriorate (If it does, replace the film).

Cleaning Up

1. Methyl salicylate is a naturally-occurring substance, as are the other esters listed under Something Extra. They can be rinsed down the drain, using a large amount of water to dilute the residual sulfuric acid. (Remember that the sulfuric acid acts a catalyst, so it was not consumed during the reaction. Therefore, whatever you put in the tube is still there.)

2. The boiling chip must not be allowed to enter the drain; it should be recovered, rinsed, and placed in the container for non-hazardous solid waste.

3. Once you are through with all of the esterifications that you plan to do, place the Pasteur pipet in the container labeled BROKEN GLASS ONLY.

4. Wash your hands before leaving the laboratory.

Analysis and Conclusions

Complete the Analysis and Conclusions section for this experiment either on your Report Sheet or in your lab report as directed by your teacher.

1. In this experiment you were interested only in the nature of the products. Explain how you would revise the experiment if the goal was to determine the percent yield. Explain completely what you would do and what measurements would be needed.

Something Extra

Any number of familiar fragrances can be made from simple, common alcohols and acids, but many of the more familiar ones involve acids or alcohols that have offensive or even toxic properties. Notable are those involving butyric acid, an extremely unpleasant smelling carboxylic acid, resulting in fragrances such as apple and pineapple.

With your teacher’s permission try some of the combinations of carboxylic acids and alcohols listed below. Do not create your own combinations without consulting your teacher. The fragrances of some common esters are listed. However, even those for which no fragrance is given will be recognizable.

The procedure for these combinations is the same as before: about 0.5 cm depth of acid, an equal volume of alcohol, and 2-3 drops of sulfuric acid catalyst are refluxed gently for several minutes. Disposal is to be carried out as described under Cleaning Up.

 Carboxylic Acid Alcohol Fragrance
 acetic acid ethyl alcohol (ethanol) ----------
 acetic acid amyl alcohol (pentyl alcohol) banana
 acetic acid octyl alcohol orange
 acetic acid n-propanol pear
 benzoic acid ethyl alcohol ----------
 formic acid isobutanol raspberry
 propionic acid isobutanol rum

 

Report for Experiment 86

Name___________________________

Section___________ Date___________

Synthesis of Esters

Prelaboratory Questions

1. Explain how the reflux condenser helps to keep the liquid reagents from boiling away during the esterification reaction.

 

 

 

 

 

 

2. Using the equation for the reaction between methanol and acetic acid as a model, write the equation for the reaction between methanol and salicylic acid. Consult Chapter 20 of your text for the structure of salicylic acid.

 

 

 

3. Suggest an explanation for the fact that esterification reactions are often referred to as condensation reactions.

 

 

 

 

Data/Observations

Reactant Initial Appearance Odor (if any)
Salicylic acid    
Methanol    

Describe what you saw during the refluxing. Could you tell that liquid was being recondensed? If so, what did you see that indicated this was happening?

 

 

 

 

 

 

 

 

Product Appearance Odor (if any)
Methyl salicylate    
Water    

Analysis and Conclusions

1. In this experiment you were interested only in the nature of the products. Explain how you would revise the experiment if the goal was to determine the percent yield. Explain completely what you would do and what measurements would be needed.

 

 

 

 

 

Something Extra

Any number of familiar fragrances can be made from simple, common alcohols and acids, but many of the more familiar ones involve acids or alcohols that have offensive or even toxic properties. Notable are those involving butyric acid, an extremely unpleasant smelling carboxylic acid, resulting in fragrances such as apple and pineapple.

With your teacher’s permission try some of the combinations of carboxylic acids and alcohols listed below. Do not create your own combinations without consulting your teacher. The fragrances of some common esters are listed. However, even those for which no fragrance is given will be recognizable.

The procedure for these combinations is the same as before: about 0.5 cm depth of acid, an equal volume of alcohol, and 2-3 drops of sulfuric acid catalyst are refluxed gently for several minutes. Disposal is to be carried out as described under Cleaning Up.

 Carboxylic Acid Alcohol Fragrance
 acetic acid ethyl alcohol (ethanol) _____
 acetic acid amyl alcohol (pentyl alcohol) banana
 acetic acid octyl alcohol orange
 acetic acid n-propanol pear
 benzoic acid ethyl alcohol _____
 formic acid isobutanol raspberry
 propionic acid isobutanol rum

 

Experiment 86  

The Synthesis of Esters

Introduction

Intent

This very quick experiment is a real treat for students because they can recognize the product as something from "real life." It is also an excellent opportunity for extra credit; you almost certainly have all the reagents needed for students to do the Something Extra. While intended for placement in Chapter 20, Organic Chemistry, it can be done at almost any time as a "real world" experiment.

Objectives

1. To synthesize one or more esters from the appropriate carboxylic acid and alcohol.

2. To identify the natural source of the ester based on recognition of its odor.

Materials (for each lab team of 2 students)

Apparatus

Reagents

Preparation Hints

The "heat transfer block" is a small aluminum block available from Laboratory Craftsmen, Beloit, WI, for use with their individual hotplates (Thermoplate™). Any hot plate will do, but if your budget permits, you might consider these. At about $70 each, they have a Off-High-Low toggle switch and can boil the water in a 250-mL beaker in about 5 minutes. One of the holes in the block just accepts a 13 mm diameter tube, giving more efficient heating. If you don't have them, the test tube can be held to the top of an ordinary hot plate or in a sand bath. Pieces of broken crucible or evaporating dish can substitute for the boiling chips. The same apparatus can be used more than once, if students do the Something Extra.

Prelaboratory Discussion

If you do this in connection with Chapter 20, take time to go through the nature of the reaction that students are to carry out. Unless you are dealing with honors students, it would be well to stay away from too much discussion of mechanism. On the other hand, if students can handle such a discussion, this is an excellent opportunity to return to catalysis and clear up a common misunderstanding about the difference between the catalyst "not being consumed" vs. not being involved. The hydrogen ion from sulfuric acid is very much involved, protonating the carbonyl oxygen, but is regenerated in the final step of the mechanism. Thus, it participates but is not consumed. Be sure that students understand the importance of the boiling stone; eruption due to superheating will spray a highly-acidic, hot mixture.

Procedure Hints

1. The Parafilm™ strip should be wrapped around enough times that the fit is snug.

2. The salicylic acid need not be weighed; the amount that fills the curved bottom and just extends into the straight-sided portion of the tube is fine. Add alcohol to a depth of about 1 cm (total).

3. If you dispense the sulfuric acid, you can check to see that the boiling stone has been added.

Disposal

No special precautions are indicated.

Postlab Discussion

Minimal discussion is needed. Students will be excited to recognize most of the esters, although they may not be able to identify them by name. Oil of wintergreen, for example, is familiar, but may get "I know I've smelled that before, but..." reaction. Athletes have the best chance, since it is in many analgesic balms.